(316g) Comparing The Mechanical Properties Of Chitosan Films Bound By Four Treatment Combinations On Implant Quality Titanium

            Titanium is commonly used as implant materials
for its mechanical properties, including strength, weight, and durability.  It
also is easily passivated and does not react with the physiological fluids
surrounding the implant.  However, titanium does not promote bone cell growth,
preventing the integration of the implant into the bone.  One way to improve
osseointegration is to coat the implant with a bioactive material, which
promotes bone cell attachment and growth [1].  Several different bioactive
materials are currently being examined, including calcium phosphate [2],
hydroxyapatite [3], and biological molecules, such as enzymes and proteins
[4-6].

            Two precursors to bone formation, hydroxyapatite
and calcium phosphate, exhibit high bioactive behavior, but are considered
ceramics.  When shear stress is applied to ceramics, the material fractures
before plastic deformation, or an irreversible change in shape, can occur [7]. 
Because ceramics cannot absorb stress, these materials are likely to crack
and/or flake when stressed during implantation, reducing the effectiveness of
the coating and preventing osseointegration of the implant.  One way to avoid
the cracking or flaking of a ceramic coating is to use a coating created from a
polymer.  Polymers are better suited to absorb the stresses applied during
implantation because of their ability to stretch, or elongate, which is a type
of plastic deformation [7]. 

            Polymers can be created both in the laboratory
and biologically.  Currently, Mississippi State University is investigating the
use of chitosan, a de-acetylated form of chitin, which is found in the
exoskeletons of shellfish and some insects [8].  Because of the ability of
invertebrates to form chitin, the polymer is the second most abundant form of
polymerized carbon found in nature, behind only cellulose [9].  Besides being
readily available, chitosan exhibits many properties that make it an ideal
material for implantation.  The material is non-toxic and the degradation
products are also non-toxic, since the by-products are part of normal cellular
metabolism [8,10].  The biopolymer is also cationic, so it attracts proteins
and cells and encourages attachment and growth of bone, a necessity for the
osseointegration of the implant [11].  Finally, chitosan has been shown to be
antibacterial and bacteriostatic [12,13].

            At Mississippi State University, four treatment
combinations have been designed to attach chitosan to implant quality
titanium.  These treatment combinations involve two different metal treatments
and two different silanation reactions [14,15].  X-Ray Photoelectron
Spectroscopy (XPS) was run on the final films, which demonstrated no
significant differences between the films produced using the four treatment
combinations [14,15].  In order to confirm that the four treatment reactions
did not affect the bulk properties of the chitosan films, mechanical tests were
performed.  Nano-indentation tests were performed to determine the hardness and
elastic modulus of the films.  Both the hardness values (0.19 ± 0.08) and the elastic modulus values (4.90 ± 1.82) showed no significant differences
between the four treatment combinations or published literature values [16].  Scanning
Electron Microscopy (SEM) of the nano-indentation marks showed that the chitosan
films could absorb the stress applied by the diamond shaped nano-indenter, as
the cracking of the film did not extend outside of the nano-indentation marks
[16].  The purpose of the four treatment combinations is to create a chitosan
film that is more strongly attached to titanium than previous research. 
Because the nano-indentation did not show any delamination events, tensile
testing was performed to determine the bond strength of the chitosan films on
the titanium surface.  While there were no significant differences between the
four treatment combinations, the results indicate that the bond strengths were
significantly higher (19.50 ± 1.63 MPa)
than previous results (1.5 MPa) [8,16].  Overall, mechanical testing showed
that the bulk properties of the chitosan films were not changed by the four
treatment combinations, but the bulk adhesion of the chitosan films was greatly
increased.

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